The development and progression of osteoarthrosis, rheumatoid arthritis and ankylosing spondylitis were determined by means of clinical, radiographic and serological examinations carried out prospectively at two-yearly intervals among adults of the Gila River Indian Community (Pima Indians) in Arizona, in conjunction with epidemiological studies of diabetes in the same community. The purpose of this investigation is to ascertain the determinants of these diseases in the population, and to identify factors which predispose to or alter the natural history of progression of the disease. Host factors such as age, sex, and various gene markers, together with various potential environmental determinants, such as obesity and evidence of exposure to infectious agents, have been investigated prospectively to determine their relationship to the development of these diseases. Longitudinal data were collected for 30 years and represent a unique data set for epidemiological studies of arthritis. Recent analyses have examined the prevalence of hip osteoarthritis in the Pima population which is somewhat greater than in the general U.S. population, and new studies of the familial aggregation of rheumatoid arthritis have been initiated.